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Functions of Bone and Skeletal System

Structure of Bone

Histology of Bone Tissue

Blood and Nerve Supply of Bone

Bone Formation

Bone’s Role in Calcium Homeostasis

Exercise and Bone Tissue

Aging and Bone Tissue

Support

Protection

Assistance in Movement

Mineral Homeostasis

Blood Cell Production

Triglyceride Storage

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Diaphysis

Epiphyses

Metaphyses

▪ Epiphyseal growth plate

Articular cartilage

Periosteum

▪ Perforating fibers

Medullary cavity

Endosteum

Long Bone Anatomy

(Humerus)

Extracellular matrix surrounding widely separated cells

Matrix

▪ 25% water

▪ 25% collagen fibers

▪ 50% crystallized mineral salts

The most abundant mineral salt is calcium phosphate

A process called calcification is initiated by bone-building cells called osteoblasts

Mineral salts are deposited and crystalize in the framework formed by the collagen fibers of the extracellular matrix

Bone’s flexibility depends on collagen fibers

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Four types of cells are present in bone tissue Osteogenic cells Undergo cell division; the resulting cells develop

into osteoblasts Osteoblasts Bone-building cells

Synthesize extracellular matrix of bone tissue Osteocytes Mature bone cells

Exchange nutrients and wastes with the blood

Osteoclasts

Release enzymes that digest the mineral components of bone matrix (resorption)

Regulate blood calcium level

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Bone is richly supplied with blood Periosteal arteries

accompanied by nerves supply the periosteum and compact bone

Epiphyseal veins carry blood away from long bones

Nerves accompany the blood vessels that supply bones The periosteum is rich in

sensory nerves sensitive to tearing or tension

The process by which bone forms is called ossification

Bone formation occurs in four situations: 1) Formation of bone in an embryo 2) Growth of bones until adulthood 3) Remodeling of bone 4) Repair of fractures

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Formation of Bone in an Embryo

Bone formation follows one of two patterns

▪ Intramembranous ossification ▪ Flat bones of the skull and mandible are formed in this way

▪ “Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull

▪ Endochondral ossification ▪ The replacement of cartilage by bone

▪ Most bones of the body are formed in this way including long bones

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Collagen fiber

Development of ossification center

Mandible

Flat bone

of skull

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bone

matrix

Development of ossification center

Calcification

Mandible

Flat bone

of skull

2

Collagen fiber

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Development of ossification center

Calcification

Mandible

Flat bone

of skull

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bone

matrix

Mesenchyme

condenses

Blood vessel

Spongy bone

trabeculae

Osteoblast

Formation of trabeculae 3

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Mesenchyme

condenses

Blood vessel

Spongy bone

trabeculae

Osteoblast

Periosteum

Spongy bone tissue

Compact bone tissue

Development of ossification center

Calcification Formation of trabeculae

Development of the periosteum

Mandible

Flat bone

of skull

3

4

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bone

matrix

1 Development of

cartilage model

Hyaline

cartilage

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

1 Development of

cartilage model

Growth of

cartilage model 2

Hyaline

cartilage

Uncalcified

matrix

Calcified

matrix

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

1 Development of

cartilage model Development of

primary ossification

center

Growth of

cartilage model 2 3

Hyaline

cartilage

Uncalcified

matrix

Calcified

matrix Nutrient

artery

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

Periosteum

Primary

ossification

center

Spongy

bone

1

Hyaline

cartilage

Calcified

matrix

Periosteum

(covering

compact bone)

Uncalcified

matrix

Calcified

matrix

Medullary

cavity

Nutrient

artery and vein

Nutrient

artery

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

Development of

cartilage model Development of

primary ossification

center

Development of

the medullary

cavity

Growth of

cartilage model

Periosteum

Primary

ossification

center

2 3 4

Spongy

bone

Uncalcified

matrix

1 Development of

cartilage model Development of

primary ossification

center

Development of

the medullary

cavity

Growth of

cartilage model 2 3 4

Hyaline

cartilage

Calcified

matrix

Periosteum

(covering

compact bone)

Uncalcified

matrix

Calcified

matrix

Medullary

cavity

Nutrient

artery and vein

Nutrient

artery

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

Periosteum

Primary

ossification

center

Secondary

ossification

center

Nutrient

artery and vein

Uncalcified

matrix

Epiphyseal

artery and

vein

Development of secondary

ossification center

5

Spongy

bone

Uncalcified

matrix

1

Articular cartilage

Spongy bone

Epiphyseal plate

Secondary

ossification

center

Nutrient

artery and vein

Uncalcified

matrix

Epiphyseal

artery and

vein

Formation of articular cartilage

and epiphyseal plate Development of secondary

ossification center

Development of

cartilage model Development of

primary ossification

center

Development of

the medullary

cavity

Growth of

cartilage model 2 3 4

5 6

Hyaline

cartilage

Uncalcified

matrix

Calcified

matrix

Periosteum

(covering

compact bone)

Uncalcified

matrix

Calcified

matrix

Medullary

cavity

Nutrient

artery and vein

Nutrient

artery

Perichondrium

Proximal

epiphysis

Distal

epiphysis

Diaphysis

Periosteum

Primary

ossification

center

Spongy

bone

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Growth in Length The growth in length of long

bones involves two major events: 1) Growth of cartilage on the

epiphyseal plate 2) Replacement of cartilage by

bone tissue in the epiphyseal plate

Osteoclasts dissolve the calcified cartilage, and osteoblasts invade

the area laying down bone matrix

The activity of the epiphyseal plate is the way bone can increase in length

At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

Growth in Thickness Bones grow in thickness at the outer surface

Remodeling of Bone Bone forms before birth and continually renews

itself The ongoing replacement of old bone tissue by

new bone tissue Old bone is continually destroyed and new bone is

formed in its place throughout an individual’s life

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A balance must exist between the actions of osteoclasts and osteoblasts

If too much new tissue is formed, the bones become

abnormally thick and heavy

Excessive loss of calcium weakens the bones, as occurs in osteoporosis

Or they may become too flexible, as in rickets and osteomalacia

Normal bone metabolism depends on several factors Minerals

Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling

Vitamins Vitamin A stimulates activity of osteoblasts Vitamin C is needed for synthesis of collagen Vitamin D helps build bone by increasing the absorption of

calcium from foods in the gastrointestinal tract into the blood

Vitamins K and B12 are also needed for synthesis of bone proteins

Hormones

During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver ▪ IGFs stimulate osteoblasts, promote cell division at the epiphyseal

plate, and enhance protein synthesis

Thyroid hormones also promote bone growth by

stimulating osteoblasts

Insulin promotes bone growth by increasing the synthesis of bone proteins

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Hormones Estrogen and testosterone cause a dramatic

effect on bone growth ▪ Cause of the sudden “growth spurt” that occurs during

the teenage year ▪ Promote changes in females, such as widening of the

pelvis ▪ Shut down growth at epiphyseal plates

Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

Fracture Types Open (compound) fracture

▪ The broken ends of the bone protrude through the skin

Closed (simple) fracture ▪ Does not break the skin

Comminuted fracture ▪ The bone is splintered, crushed, or broken into pieces

Greenstick fracture ▪ A partial fracture in which one side of the bone is broken and the other side bends

Impacted fracture ▪ One end of the fractured bone is forcefully driven into another

Pott’s fracture ▪ Fracture of the fibula, with injury of the tibial articulation

Colles’ fracture ▪ A fracture of the radius in which the distal fragment is displaced

Stress fracture ▪ A series of microscopic fissures in bone

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Compact bone Spongy bone

Periosteum

Fracture hematoma

Fracture hematoma

Bone fragment

Osteocyte

Red blood cell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginous callus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation 2

Compact bone Spongy bone

Periosteum

Fracture hematoma

Fracture hematoma

Bone fragment

Osteocyte

Red blood cell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Bony callus

Spongy bone

Osteoblast

Bony callus formation

Osteocyte

3

Compact bone Spongy bone

Periosteum

Fracture hematoma

Fracture hematoma

Bone fragment

Osteocyte

Red blood cell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginous callus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation 2

Spongy bone

Osteoblast

Osteoclast

New compact bone

Bony callus formation Bone remodeling

Osteocyte

3 4

Compact bone Spongy bone

Periosteum

Fracture hematoma

Fracture hematoma

Bone fragment

Osteocyte

Red blood cell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginous callus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation 2

Bony callus

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Bone is the body’s major calcium reservoir Levels of calcium in the blood are maintained by

controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone Both nerve and muscle cells depend on calcium

ions (Ca2+) to function properly Blood clotting also requires Ca2+

Many enzymes require Ca2+ as a cofactor

Actions that work to decrease blood Ca2+ level

The thyroid gland secretes calcitonin (CT) which

inhibits activity of osteoclasts

The result is that CT promotes bone formation and decreases blood Ca2+ level

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Bone tissue alters its strength in response to changes in mechanical stress Under stress, bone tissue becomes stronger through

deposition of mineral salts and production of collagen fibers by osteoblasts

Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers

The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity

Weight-bearing activities help build and retain bone mass

The level of sex hormones diminishes during middle age, especially in women after menopause A decrease in bone mass occurs Bone resorption by osteoclasts outpaces bone

deposition by osteoblasts Female bones generally are smaller and less

massive than males Loss of bone mass in old age has a greater

adverse effect in females

There are two principal effects of aging on bone tissue:

1) Loss of bone mass

▪ Results from the loss of calcium from bone matrix

▪ The loss of calcium from bones is one of the symptoms in osteoporosis

2) Brittleness

▪ Results from a decreased rate of protein synthesis

▪ Collagen fibers gives bone its tensile strength

▪ The loss of tensile strength causes the bones to become very brittle and susceptible to fracture

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